http://old.openspf.org/index.html is a great little site. To quote the site:
SPF is Sender Policy Framework
SPF fights return-path address
forgery and makes it easier to identify
Domain owners identify sending mail servers in DNS.
SMTP receivers verify the envelope sender address against
this information, and can distinguish authentic messages from
forgeries before any message data is transmitted.
Cross posted from my blog at http://schema.sol3.net/kbarrows